1. Field of the Invention
The present invention relates to a radio base station and a method of controlling radio communications. More particularly, the present invention relates to a radio base station and a method of controlling radio communications which use transmit diversity in TDMA or TDD systems.
2. Description of the Related Art
Fading typically occurs in radio communications, greatly degrading transmission quality, that is, bit error rate characteristics.
Transmit diversity is a known method for compensating such degradation of transmission quality due to fading. Downlink transmit diversity, a kind of transmit diversity, will be described.
FIG. 1 partially illustrates the configuration of a radio base station 50 using a conventional downlink transmit diversity. The radio base station 50 includes, as shown in FIG. 1, an encoder 51, a spreader 52, weighting units 531 and 532, filters 541 and 542, transmitters 551 and 552, antennas 561 and 562, and an uplink channel estimator 57.
The encoder 51 is connected to the spreader 52, encoding and interleaving data to be transmitted (hereinafter referred to as transmission data) under a predetermined scheme and transmitting the encoded and interleaved transmission data to the spreader 52.
The spreader 52 is connected to the encoder 51 and the weighting units 531 and 532, spreading and scrambling transmission data received from the encoder 51 under a predetermined scheme and transmitting the spread and scrambled transmission data to the weighting units 531 and 532.
The weighting units 531 and 532 are connected to the spreader 52, the filters 541 and 542, and the uplink channel estimator 57, and weighting (e.g., multiplying) transmission data received from the spreader 52 by weighting factors W1 and W2 received from the uplink channel estimator 57. The weighting units 531 and 532 transmit the weighted transmission data to the filters 541 and 542.
The filters 541 and 542 are connected to the weighting units 531 and 532 and the transmitters 551 and 552, filtering transmission data received from the weighting units 531 and 532 and transmitting the filtered transmission data to the transmitters 551 and 552.
The transmitters 551 and 552 are connected to the filters 541 and 542 and the antennas 561 and 562, transmitting transmission data received from the filters 541 and 542 via radio lines with predetermined carrier frequencies, in cooperation with the antennas 561 and 562.
The uplink channel estimator 57 is connected to the weighting units 531 and 532 and the antennas 561 and 562, monitoring uplink radio communications channels (e.g., time slots, spread codes or carrier frequencies) set for the antennas 561 and 562, thereby estimating the status of the uplink radio communications channels (e.g., waveform distortion, delay fluctuation, amplitude fluctuation and phase lag) (performing channel estimation), determining the weighting factors W1 and W2 based on the channel estimation, and transmitting the determined weighting factors W1 and W2 to the weighting units 531 and 532 .
STD (selective transmit diversity), a kind of downlink transmit diversity, for example, uses “1” or “0” as the weighting factor W1 and correspondingly “0” or “1” as the weighting factor W2. As a result, transmission data is transmitted only via either a first transmission line consisting of the filter 541, transmitter 551 and antenna 561 or a second transmission line consisting of the filter 542, transmitter 552 and antenna 562.
TxAA (Transmission Adaptive Array), a kind of downlink transmit diversity, can assign weighting factors to the antennas 561 and 562, respectively.
FIGS. 2(a) and 2(b) illustrate time slot configurations used on TDD radio communications channels.
When, for example, time slots shown in FIG. 2(a) are used, the uplink channel estimator 57 causes down link time slot #3 to reflect channel estimation of uplink time slot #2 and causes downlink time slot #5 to reflect channel estimation of uplink time slot #4, thus controlling downlink transmit diversity based on the channel estimation of immediately preceding uplink time slots.
When time slots shown in FIG. 2(b) are used, for example, the uplink channel estimator 57 controls downlink transmit diversity so that channel estimation of uplink time slot #2 is reflected by all of the subsequent time slots (#3 to #15).
In a radio base station and a method of controlling radio communications using the conventional downlink transmit diversity, there are time differences between the instant of channel estimation of uplink time slots and the instant of transmission of transmission data via downlink time slots. Directly using the channel estimation of uplink time slots at the time of transmitting transmission data via downlink time slots may result in estimation errors.
In particular, as in the case of FIG. 2(b), the greater the time difference between the instant of channel estimation of an uplink time slot and the instant of transmission of transmission data via a downlink time slot, the greater an estimation error.
Thus when the time difference is great, a method of controlling radio communications and a radio base station using the conventional downlink transmit diversity may degrade the transmission quality characteristics more than without using the downlink transmit diversity, as shown in FIG. 3.
In an art of reducing the size of receivers on the assumption that channel estimation results of different users are the same, such as “SUD (single user detection)”, the use of transmit diversity causes different results of channel estimation of different users, thus degrading transmission quality characteristics.